When an engine performs combustion in an excess oxygen atmosphere, an exhaust gas containing carbon monoxide, hydrocarbon, nitrogen oxide (NOx), etc., is discharged from the engine. As a catalyst for reducing NOx which is discharged in an oxygen excess atmosphere, by using a reducing agent such as ammonia, a catalyst of a Selective Catalytic Reduction (SCR) type has been known.
Under these circumstances, Patent Document 1 describes, for example, a catalyst having supported therein titanium hydroxide, tungstic acid or salts thereof, cerium dioxide, etc., for removing nitrogen oxide by using ammonia as a reducing agent (e.g., claims 1, 8 and 9 of Patent Document 1).